This is Android N freeform window mode and you can now enable it !

Recently Google released Android N developer preview with many new features including Split screen , night mode and more .

Few days back ars technica found a code of Android N freeform window mode ( Use Actual multiple windows at the same time like remix os ) , but the feature was not there in Android N . But now you can enable this awesome feature if you are using Android N developer preview .

You can even use freeform window in split screen too , though it will use much RAM and might lag in many devices with low RAM . May be that's the reason Google hid this feature . But you can still enable it by doing a little work .